1. Field of the Invention
The present invention relates to a process for producing austenitic stainless steels which process prevents occurrence of rolling defects, particularly slivers.
2. Description of Prior Arts
It has been conventionally known that austenitic-alloys such as austenitic stainless steels show inferior hot workabilities during break-down rolling of ingots, for example, thus susceptible to cracks, and higher alloy steels are more difficult to work from a large steel ingot. Up-to-now, various studies have been made for overcoming these difficulties, and effective means for preventing cracks even in the case of high alloy steels have been made and it is now very seldom that the high alloy steels cannot be satisfactorily produced due to occurrence of large cracks, but they are still susceptible to small cracks.
Meanwhile, as continuous casting technics have been adopted more and more, similar problems have been found in hot workability of continuously cast slabs into thick plates and these plates are often susceptible to occurrence of small crack defects, particularly slivers as will be described hereinafter.
Therefore, separate from the problem of large cracks which prohibit commercial production of the steel plates, there are problems to be solved that when a solidified steel structure, such as a continuous cast steel slab and a steel ingot, is hot rolled and acid-pickled, fine cracks normally called slivers occur locally to a very shallow depth on the surface of the steel after acid-pickling. These slivers naturally lower the production yield of the steel plates, and require a re-conditioning step, and in the worst case resultant products fail to meet ordered size specifications, thus being rejected.
The present inventors have made investigations for determining causes for these fine defects and slivers (hereinafter called "slivers"), and have found that these slivers are to be a kind of hot working crack due to inferior hot workability. Although these slivers do not cause a vital problem which prohibits commercial production with respect to the hot workability, these cracks must be considered to be a practically great problem, because these cracks are found in steel grades, such as SUS (Japanese Industrial Standard) 304, SUS 316 and SUS 347 stainless steels which are normally produced on a mass-production scale. Therefore the present inventors have investigated on possible causes of the cracks and studied on possibility of quantitative means for effectively and economically preventing the cracks.
For example, when a continuously cast slab of SUS 316 grade is ground 2 mm on the whole surfaces, heated to 1200.degree. C. or higher and rolled, slivers in the worst case occur all over the surfaces of the acid-pickled steel sheets, chiefly on the edge sides of about 150 mm in width, and reach 2 to 5 mm depth max. condensing in a great number. In order to obtain satisfactory final products, these defect portions must be totally ground. These slivers sometimes occur also in hot coils.
The present inventors have made detailed investigations on these phenomena so as to determine the nature of slivers and their causes for different steel grades and different production processes, and found the following facts:
(1) Slivers are caused due to the lowering of deformation ability of the steel during hot working, and occur around the .gamma. grains during soldification.
(2) The occurrence of slivers is greatly influenced by production conditions, such as casting conditions and hot rolling conditions.
(3) Mainly sulfides segregate at the .gamma. grain boundaries, and cracks develop along these sulfides.
Therefore, preventive means are desired for preventing the sulfides from precipitating at the .gamma. grain boundaries, and for this purpose it is necessary to maintain the contents of impurities, such as sulfur, below their permissible limits. Thus the sulfur content should be maintained not larger than 0.005% and oxygen not larger than 0.006%.
Further, at the time of solidification of the steel, it is desirable to disperse the impurities, such as sulfur and oxygen, and utilize the proeutetic delta ferrite as an effective means for preventing the precipitation of the impurities at the grain boundaries, and for this purpose the ".delta.cal(%)" index representing the amount of the delta ferrite after the solidification of the steel should be desirably between -3 and 4. EQU .delta.cal(%)=3(Cr+Mo+1.5 Si+0.5 Nb)-2.8(Ni+1/2Mn+1/2Mn+1/2Cu)-84 (C+N)-19.8.
The results of our investigation on the relation between the occurrence and the continuous casting conditions as well as the hot rolling conditions have revealed that the casting temperature control with respect to the continuous casting and the heating temperature control with respect to the hot rolling are very important for completely eliminating the occurrence of slivers. Thus, suppose the nitrogen content in the alloy is N(%) by weight, and the casting temperature is expressed as the difference .DELTA.T(.degree.C.) between the molten metal temperature T in the tundish (tundish temperature) and the melting point T.sub.L of the alloy steel, and the tundish temperature is controlled under the condition of N.times..DELTA.T.ltoreq.1.5. Continuously cast steel slabs obtained under this condition have been found to have very little susceptibility to the occurrence of slivers during the subsequent hot rolling.
Further, the present inventors have found it is very effective for preventing the occurrence of slivers to control the heating temperature H.sub.T for hot rolling under the condition of H.sub.T (.degree.C.).ltoreq.1325(.degree.C.)-50.times.[N.times..DELTA.T].